NMR apparatus and method utilizing pulsed static magnetic fields

ABSTRACT

An apparatus and method for making NMR measurements uses an electromagnet for producing the static magnetic field. When used in well logging applications, the absence of a strong permanent magnet eliminates almost completely the amount of ferromagnetic debris picked up on the logging tool when passed through casing. The absence of debris results in the static magnetic field being substantially in conformance with design. The electromagnet is designed to give a static field of 0.6-6 mT (10-100 times the earth&#39;s magnetic field). Free induction decay or spin echo measurements may be made to give low resolution measurements of bulk properties of earth formations. The same coil configuration may be used to estimate body fat measurements of a human body. An alternate embodiment of the invention uses a capacitive discharge through an electromagnet with a time varying magnetic field with the receiver operating at a different frequency from the transmitter.

FIELD OF THE INVENTION

[0001] The invention is related to the field of Nuclear MagneticResonance (NMR) apparatus and methods. Specifically, the inventionrelates to NMR apparatus and methods using pulsed static magneticfields.

BACKGROUND OF THE INVENTION

[0002] When hydrogen nuclei are placed in an applied static magneticfield, a small majority of spins are aligned with the applied field inthe lower energy state, since the lower energy state in more stable thanthe higher energy state. The individual spins precess about the appliedstatic magnetic field at a resonance frequency also termed as Larmorfrequency. This frequency is characteristic to a particular nucleus andproportional to the applied static magnetic field. An alternatingmagnetic field at the resonance frequency in the Radio Frequency (RF)range, applied by a transmitting antenna to a subject or specimen in thestatic magnetic field flips nuclear spins from the lower energy state tothe higher energy state. When the alternating field is turned off, thenuclei return to the equilibrium state with emission of energy at thesame frequency as that of the stimulating alternating magnetic field.This RF energy is generating an oscillating voltage in a receiverantenna whose amplitude and electronic rate of decay depend on thephysicochemical properties of the tissue and the magnetic environment ofthe nuclei. The applied RF field is designed to perturb the thermalequilibrium of the magnetized nuclear spins, and the time dependence ofthe emitted energy is determine by the manner in which this system ofspins return to equilibrium magnetization. The return is characterizedby two parameters: T₁, the longitudinal or spin-lattice relaxation time;and T₂, the transverse or spin-spin relaxation time.

[0003] There are at least two applications in which samples volumes aresubstantial and bulk material properties are of interest. One of theseis logging of wells drilled for hydrocarbon recovery from earthformations and another is whole body fat determination.

[0004] Measurements NMR parameters of fluid filling the pore spaces ofthe earth formations such as relaxation times of the hydrogen spins,diffusion coefficient and/or the hydrogen density is the bases for NMRwell logging. NMR well logging instruments can be used for determiningproperties of earth formations including the fractional volume of porespace and the fractional volume of mobile fluid filling the pore spacesof the earth formations.

[0005] Pulsed RF magnetic fields are imparted to the material underinvestigation to momentarily re-orient the nuclear magnetic spins of thehydrogen nuclei. RF signals are generated by the hydrogen nuclei as theyspin about their axes due to precession of the spin axes. The amplitude,duration and spatial distribution of these RF signals are related toproperties of the material under investigation. In the well loggingenvironment, contrast is high between free and bound fluids based ontheir relaxation times, between oil and water based on their relaxationtimes and diffusion coefficient. In medical applications, tissuecontrast is high between fat and muscle based on their relaxation timesand can be further enhanced by application of certain RF sequences.

[0006] Methods of using NMR measurements for determining the fractionalvolume of pore space and the fractional volume of mobile fluid aredescribed, for example, in Spin Echo Magnetic Resonance Logging:Porosity and Free Fluid Index Determination, M. N. Miller et al, Societyof Petroleum Engineers paper no. 20561, Richardson, TX, 1990. In porousmedia there is a significant difference in T₁ and T₂ relaxation timespectrum of fluids mixture filling the pore space. For example, lighthydrocarbons and gas may have T₁ relaxation time of about severalseconds, while T₂ may be three orders of magnitude smaller. Thisphenomenon is due to diffusion effects in the presence of gradients inthe static magnetic field. The gradients may be external (from theapplied static field) or internal. Internal magnetic field magnitudegradients are due to differences in magnetic susceptibility between therock matrix of the formation and the fluids in the pores of the matrix.

[0007] Power requirements in NMR oil well logging have to be optimizedfor high efficiency operation. In order to perform a valid NMRexperiment, a substance should be polarized for about 5 times thelongest T₁ relaxation time, which is about 1 second long. TypicalCarr-Purcell-Meiboom-Gill (CPMG) pulse sequences are about 0.5 to 1second long. However, because of low signal-to-noise ratio (SNR),several repetitions of a CPMG sequence are required to bet an adequateSNR.

[0008] The earliest NMR logging instruments used the earth's magneticfield for providing the static field for NMR measurements. See, forexample, U.S. Pat. No. 3,004,212 to Coolidge et al; U.S. Pat. No.3,188,556 to Worthington; U.S. Pat. No. 3,538,429 to Baker; and U.S.Pat. No. 2,999,204 to Jones et al. The earth's magnetic field isapproximately 60 μT at the poles with a Larmor frequency f for protonsof approximately 2.5 kHz. The signal level per unit volume for an NMRsurvey is approximately proportional to f⁷ ⁴. The early NMR logginginstruments suffered from the problem of low resolution because signalsfrom a large volume of the earth were required to get an acceptable SNR.When the earth's magnetic field is used for the static field, there isno problem in having a uniform static field over a large region, so thatSNR is not a major problem; however, there are many applications inwhich high resolution is required. This is difficult to achieve usingthe earth's magnetic field as the static field for NMR experiments.

[0009] In order to achieve high resolution, NMR devices used in recentyears for well logging operations use permanent magnets to generate thestatic magnetic field. These devices typically operate at 1 MHzcorresponding to a magnetic field in the region of investigation of0.0235T. Needless to say, this requires the use of permanent magnetswith a strong magnetic field as part of the logging instrument.

[0010] For example, U.S. Pat. No. 4,350,955 to Jackson et al discloses apair of permanent magnets arranged axially within the borehole so theirfields oppose, producing a region near the plane perpendicular to theaxis, midway between the sources, where the radial component of thefield goes through a maximum. Near the maximum, the field is homogeneousover a toroidal zone centered around the borehole. U.S. Pat. No.4,717,877 to Taicher et al teaches the use of elongated cylindricalpermanent magnets in which the poles are on opposite curved faces of themagnet. The static field from such a magnet is like that of a dipolecentered on the geometric axis of the elongated magnets and provides aregion of examination that is elongated parallel to the borehole axis.The RF coil in the Taicher device is also a dipole antenna with itscenter coincident with the geometric axis of the magnet, therebyproviding orthogonality of the static and magnetic field over a full360° azimuth around the borehole. U.S. Pat. No. 6,023,164 to Prammerdiscloses a variation of the Taicher patent in which the tool isoperated eccentrically within the borehole. In the Prammer device, NMRlogging probe is provided with a sleeve having a semi-circular RF shieldcovering one of the poles of the magnet: the shield blocks signals fromone side of the probe.

[0011] These, and others too numerous to mention, have been used forwireline logging wherein the logging tool is conveyed on a wireline intoa borehole, as well as Measurement-While-Drilling (MWD) operations wherethe logging tool forms part of the drilling assembly. All of these toolstypically have a region of investigation no more than a few centimetersinto the formation and a few millimeters in thickness. Repeatability ofthe observations requires that the static magnetic field be predictableto a high level of accuracy. An unappreciated problem in NMR logging ofearth formations using strong permanent magnets is that the staticmagnetic field in the subsurface may not correspond to that expected onthe basis of the design of the magnet. This is due to the fact thelogging instruments, whether on a wireline or as part of an MWDapparatus, have to pass through several hundreds or thousands of metersof casing that is used to line boreholes. To understand the consequencesof this, a brief review of the process of drilling wells is needed.

[0012] In the drilling of oil and gas wells, drill bits and otherequipment are attached to a drill string for boring a hole into theearth. Typically, a drill string may comprise a long string of manyconnected sections of drill pipe which extend from the earth's surfacedown into the wellbore or hole being formed by a drill bit connected atthe bottom end of the drill string. As the wellbore penetrates moredeeply into the earth, it becomes increasingly desirable to installcasing in the wellbore, running down from the surface.

[0013] Casing is placed in the wellbore for one of two reasons. Thefirst may be to prevent the wall of the wellbore from caving in duringdrilling and to prevent seepage of fluids from the surrounding stratainto the wellbore. Casing is absolutely essential when drilling throughan overpressured section (with an abnormally high fluid pressurerequiring heavy drilling muds) into a normally pressured orunderpressured section below: in such situations, casing is set afterdrilling through the overpressured formation and the mud weight isreduced. A second reason may be to prevent damage to the reservoir rocksby the drilling mud in the borehole forcing its way into the formation.Even in normal drilling, it is common to set casing of several differentsizes in the borehole.

[0014] During rotary drilling operations drill strings are subjected toshock, abrasion and frictional forces which are exerted on the drillstring whenever the drill string comes in contact with the walls of thewellbore or casing. Both the drillstring and the casing are usually madeof steel, a ferromagnetic material, so that the abrasion forces willresult in large quantities of ferromagnetic debris within the casing.There are numerous methods and devices for reducing the abrasion. Noneof them can be completely effective. Circulating drilling mud duringdrilling is quite effective in bringing cuttings from the formation tothe surface but is not effective in completely flushing the more densemetallic debris out of the borehole.

[0015] As a result of this, when an NMR logging tool, whether on awireline or as part of an MWD apparatus, is conveyed into a boreholethrough casing, much of the magnetic debris within the casing willattach to the tool. This can distort the static magnetic field producedby the permanent magnets in an unpredictable manner. In addition, sincethe RF pulses are produced by transmitter coils on the logging tool, theRF field is also distorted. Compounding the problem is the fact that thespin-echo signals also have to pass through this debris. U.S. Pat. No.5,451,873 to Freedman et al. teaches a method of calibrating an NMR toolto account for the accumulation of magnetic debris on the tool. For aso-called “saddle point” tool used in Freedman, one effect of the debrisis to change the static field (and hence the Larmor frequency) in theregion of investigation. Freedman makes a one-time adjustment to thetool frequency prior to using the tool. The frequency shift is notnecessary for gradient tools since for a fixed frequency, the volume ofinvestigation changes. A continuing problem remains: how to compensatefor time varying effects of the debris.

[0016] In addition to the signal distortion, there is also the practicalproblem of conveying a strongly magnetized logging tool several meterslong through a ferromagnetic casing. This problem is exacerbated indeviated or horizontal boreholes.

SUMMARY OF THE INVENTION

[0017] In one embodiment, the present invention is a method for nuclearmagnetic resonance (NMR) sensing of earth formations. An electromagneton a logging tool is used to induce a static magnetic field forpolarization of nuclei within a region of the earth formations. A radiofrequency pulse is used to tip the magnetic spins of the nuclei. Areceiver is used to measure either the free induction decay or spin echosignals (using a CPMG pulse sequence) from the precessing nuclei. Thewait time between the activation of the electromagnet and the initial RFpulse is related to a T₁ of the formations. When the static magneticfield strength is 10-100 times that of the earth's field, it is possibleto obtain low resolution estimates of properties of large volumes ofearth formation. The logging tool may be conveyed into the earth on awireline or on a drilling tubular.

[0018] In an alternate embodiment of the invention, a time varyingstatic field is produced using an electromagnet. The transmitter and thereceiver operate at different frequencies. This reduces the ringingsignals in the receiver and, after calibration, provides a measurementof bulk composition

[0019] Another embodiment of the invention may be used for estimatingfat composition of a human body. A prior art MRI device is operated inaccordance with a method of present invention to provide a low intensitystatic magnetic field, making it possible to obtain low cost body fatand lean measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic illustration of an NMR tool of the presentinvention deployed in a borehole.

[0021]FIG. 2 is a schematic illustration of a cross-section of anembodiment of a logging device of the present invention.

[0022]FIG. 3 shows an alternate embodiment of the present inventionincluding an arrangement of the electromagnet coil, transceiver coil andadditional receiver coil for use in logging of wellbores.

[0023]FIG. 4 is a schematic illustration of a conventional NMRmeasurement method.

[0024]FIG. 5 is a schematic illustration of a method of making NMRmeasurements in a time-varying static magnetic field.

[0025]FIG. 6 is an illustration of the pulse sequences for a method ofmaking NMR measurements in a time-varying static magnetic field.

[0026]FIG. 7 shows an apparatus for making measurements of whole bodycomposition.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring now to FIG. 1, there is shown an NMR well logging tool10 conveyed in a borehole 20 within earth formations 30. For exemplarypurposes, the tool is shown conveyed by a wireline 40. Surface equipmentshown at 50 can be of a conventional type and includes a processor thatcommunicates with the downhole equipment. The deployment on a wirelineis for illustrative purposes only and the present invention may also beused in Measurement-while-Drilling (MWD) and Logging while tripping(LWT) environments using known prior art configurations includingdrilling tubulars such as a drillstring or coiled tubing.

[0028] As shown in FIG. 1, the tool 10 has a pair of coils, a polarizingcoil 3 that forms an electromagnet and an excitation coil 5 wound on anon-conductive core (not shown). In a preferred embodiment of theinvention, an axis 4 of the coil 3, referred to as the polarizing coil,is substantially perpendicular to a longitudinal axis 12 of the boreholeand an axis 6 of the coil 5, referred to as the excitation coil, issubstantially perpendicular to the longitudinal axis of the borehole 12and to the axis 4 of the polarizing coil 3. FIG. 2 schematicallyillustrates a cross-section of an embodiment of a logging device of thepresent invention. The polarizing coil 3 is preferably consists ofmultiple winding, whereas the excitation coil 5 preferably consists ofone or few winding only. This is shown in FIG. 2.

[0029] When an electrical current is passed through the polarizing coil3, this produces a static magnetic field in the earth formation 30 inthe vicinity of the tool that is substantially perpendicular to theborehole axis 12. In one mode of operation, the current in thepolarizing coil is kept on for a time equal to a few times the largestT₁ value of fluids in the earth formation. Typically, the duration ofthe polarizing pulse is several seconds. As a result of this polarizingpulse, nuclear spins in the earth formation 30 will become re-orientedsubstantially parallel to the polarizing field and substantiallyperpendicular to the borehole axis 12.

[0030] The NMR well logging tool 10 further includes the previouslydescribed excitation coil 5, (further referred to as a transmitter,receiver or transceiver coil) which can comprise one or more coilwindings. Radio Frequency (RF) alternating current passing through thecoil generates an RF magnetic field in the earth formation 30substantially perpendicular to the static magnetic field. The freeprecession of the nuclear magnetic moments around the static magneticfield direction induce an RF signal in the receiver antenna 5. Such amagnetic field arrangement is conventional for NMR experiments. The RFmagnetic field may be modulated by a modulating signal that comprises atleast one pulse. When a single pulse with a 90° tipping angle is usedand the duration of the polarizing pulse is around T₁ (few millisecondsto few seconds), the amplitude of the signal in the receiver antenna isindicative of T₁ Alternatively, the at least one pulse may be a sequenceof pulses such as a CPMG sequence or a modified CPMG sequence as taughtin U.S. Pat. No. 6,163,153 to Reiderman, the contents of which areincorporated herein by reference. Excitation coil 5 is preferablyelongated along the longitudinal axis 12 of the borehole and is severaltimes longer than diameter of the borehole 10. In this case this coilgenerate a substantially two-dimensional magnetic field within theformation of interest. Such a field is perpendicular to the longitudinalaxis 12 at any point within the formation of interest.

[0031] In an alternate embodiment of the invention shown in FIG. 3, apolarizing coil 21 in a form of a solenoid electromagnet produces astatic magnetic field in the earth formation 30 in the vicinity of thetool that is substantially parallel to the borehole axis 12.Additionally, instead of a single transceiver coil 5, a pair oforthogonal coils 22 and 23 are used with coil axes orthogonal to eachother and to the borehole axis 12. Alternatively, the device of FIG. 3may be used so that coils 22 and 23 are used both as a transmitter and areceiver. One possibility is to use the coil 23 only as a receiver. Thesecond possibility is to use the coil 22 also as receiver to detect anadditional component of the spin-echo signals, which is orthogonal tothe signal component received in the first receiving coil. Theorthogonality of the two coil axes to the axis of the polarizing coil 21substantially reduces the current induced in the transmitter/receivercoil by the termination of the polarizing pulse. When an electricalcurrent is passed through the polarizing coil 21, this produces a staticmagnetic field in the earth formation 30 in the vicinity of the toolthat is substantially parallel to the borehole axis 12.

[0032] The tool of FIGS. 1-3 may be easily passed through casing withoutaccumulation of ferromagnetic debris or sticking caused by magneticattraction to well casing, as there is no permanent magnet on the tool.Low magnetic fields (0.6-6 mT, i.e., 10-100 times the earth's field) areeasy to generate by DC currents. In the logging environment and in thecase of whole body composition analysis the region of investigation islarge so that the SNR is expected to be adequate with measurement timesof a few seconds (i.e., with a few repetitions of the CPMG sequence).For that short time a pulse of DC current (low average powerrequirements) may generate the static magnetic field. Optionally, acapacitor may be used on the logging tool with the capacitor dischargeproviding the static field. Yet another option is to generate a pulse ofstatic magnetic field only during one sequence.

[0033] In an alternate embodiment of the invention, the excitation coilis pulsed after partial polarization of the nuclear spins. The partialpolarization time (time between beginning of polarizing pulse andbeginning of excitation RF pulse) may range from 0.1 to 5 times thelargest value of T₁ in the formation. By making such measurements withseveral different partial polarization times, information may beobtained regarding the T₁ distribution of the formation.

[0034] Conventional NMR measurements are made by applying apredetermined static magnetic field to the region under investigation topartially or fully polarize the nuclear spins. Following this, a RFmagnetic field is applied to determine the decay characteristics ofnuclear spins. The NMR experiment may involve measurement of freeinduction decay (FID) or it may involve spin-echo measurements. Forexample, in a commonly used method of spin-echo measurements, the RFmagnetic field comprises a tipping pulse that tips the nuclear spins by90° and starting a precession of the spins. A series of refocusingpulses is applied and pulse echo signals are measured using a receivercoil. The transmitter and receiver coils may be the same. A conventionalNMR method is illustrated in FIG. 4 where the abscissa 101 is time, theordinate 103 is the magnetic intensity shown by 105 and the measurementsare made during a time duration 107. Regardless of whether FID orspin-echo measurements are made, there are stringent requirements on thestability of the magnetic field (typically 1%) and its gradient.Additionally, if the same antenna is used for transmitting and receivingthe RF signals, switching transients may be present. This, and ringingproduced by the initial tipping pulse causes problems in making accuratemeasurements of signal amplitudes. Many of these problems are avoided inthe novel procedure described next.

[0035]FIG. 5 illustrates the methodology of the present invention. Aslowly varying magnetic field is produced by passing a current through apolarizing coil, the field intensity as a function of time being shownby 205. The RF field is applied at one time interval 207 with a RFfrequency corresponding to magnetic field intensity 208. The staticmagnetic field produced by the polarizing coil may then increase to somemaximum value and then starts decreasing. The measurements are madeduring a time interval such as 209 when the field intensity 210 (and thecorresponding precession frequency) are different from those of thetipping pulse. The advantage of this is obvious since this eliminatesthe ringing. In addition, stability requirements on the static field aregreatly reduced, the main requirement being that the measurements berepeatable. There is no requirement that the nuclear spins be fullypolarized, or even that the extent of the partial polarization be known.Furthermore, there is no requirement that the RF pulse tip the nuclearspins by 90°, the only requirement being that of repeatability.Different materials will respond differently and can be calibratedaccordingly. For example, water, oil, and gas may have differentresponses. Once a calibration is performed, the pulsed polarizationmethod may be used for logging of earth formations. This method may beused with the coil configurations described above with reference toFIGS. 1-3.

[0036] Those versed in the art would recognize that the temporalvariation of the static field such as that shown in FIG. 5 can be easilyobtained by discharge of a capacitor. For a given coil configuration,the requirements of stability are easily met.

[0037] When static magnetic field spatial distribution at any moment intime is homogenous, it is possible to make FID measurements. Thecondition of homogeneity is applicable for some cases of whole bodycomposition determination. However, in the logging environment, staticmagnetic field gradients exist and therefore, a spin-echo measurementhave to be implemented. FIG. 6 shows the methodology of performing aspin-echo experiment with a time-varying static magnetic field. Thefield intensity of the static magnetic field as a function of time beingshown by 305. A first pulse of RF magnetic field is applied at a firsttime interval 307 with a RF frequency corresponding to magnetic fieldintensity 306. The RF field intensity and the duration of this pulse areset to be such, that the nuclear spins are tipped by approximately 90°.The static magnetic field produced by the polarizing coil may thenchanged to a higher or lower level. FIG. 6 shows the static field beingincreased. A second pulse of RF field is applied at a second timeinterval 309 with a RF frequency corresponding to static magnetic fieldintensity 308. The RF magnetic field intensity and the duration of thispulse are set so that the nuclear spins are tipped by approximately180°. Since the static magnetic field is inhomogeneous, the timing ofthe 90° and the 180° pulses is important. The measurements are madeduring a time interval such as 311 when the field intensity 310 (and thecorresponding precession frequency) are different from those of thetipping and the refocusing pulses. Proper timing of the tipping andrefocusing pulses is important due to the field variation.

[0038] At each point in space the static magnetic field is related tothe electrical current which generate the field. For the NMR welllogging equipment shown in FIGS. 1-3, the regions of equal staticmagnetic field form cylindrical shells. Therefore, if the first pulse ofRF magnetic field is applied at time interval different from timeinterval 307 with a RF frequency corresponding to magnetic fieldintensity 306, nuclear spins at different shell will be tipped. Timingof the 90° and the 180° can be easily calibrated by adjusting receivedsignal to maximum amplitude.

[0039] The method has been described above using a single refocusingpulse followed by the reception of a single echo. The method may begeneralized to include a plurality of refocusing pulses with a varyingtime interval therebetween and receiving a corresponding set of echos.Additionally, measurements made at different depths in a borehole may bedeconvolved using known signal processing techniques to improve theresolution of the estimated properties of the formation.

[0040] The magnet and coil arrangement and the pulsing method describedabove may also be used for medical applications. Instruments used forMagnetic Resonance Imaging (MRI) require very strong main magnets inaddition to a variety of smaller gradient fields. Early NMR instrumentsused resistive electromagnets to produce a main field of the order of0.05 to 0.14 T. The limitation on magnetic field strength is the powerdissipation of the resistive magnet. To overcome this problem, trappedfield (superconductive) magnets were introduced to supply the mainfield. Although these provided the required high fields withoutdissipation problems, they are very expensive, difficult to operate andconsume relatively large amounts of liquid helium refrigerant. They dohowever provide strong fields of the order of a couple of Tesla.

[0041] In high-resolution NMR technique or Magnetic ResonanceSpectroscopy different proton resonance frequencies are observed forprotons in chemically different environments. The magnetic fieldstrength experience by these protons is usually less than that of theapplied field due to diamagnetic shielding caused by the motion of theirvalence electrons and those of adjacent atoms in response to the appliedmagnetic field. These differences are called chemical shift and areusually measured with respect to some reference standard. Water can beeasily distinguish from oils and fats based on NMR high-resolutionspectra. However, this technique is expensive and only small volumes maybe tested. Low-resolution NMR technique applied to mixture of water andoil will exhibit spectrum of NMR signal containing one single “wideline” at the weighted mean proton resonance frequency. This technique isreferred in the literature as “wide-line” NMR.

[0042] U.S. Pat. No. 4,521,734 to Macovski describes a method forimaging a region of the body using nuclear magnetic resonance, where themain magnet has controllable field amplitude. This imaging methodconsist of the following steps: applying gradient fields; increasing theamplitude of the main magnetic field during the image data acquisitiontime interval including the excitation and reception intervals for atime shorter than the thermal time constant of the main magnet;decreasing the amplitude of the main magnetic field during times thatimage data is not being acquired; and processing the nuclear magneticresonance signals to provide spatial information for creating an image.Usually resistive magnets are operational at relatively low magneticfield, which from signal-to-noise ratio is problematic in hydrogenimaging but impossible for phosphorus. Magnetic field strength isrestricted by the power dissipated in the magnet. Macovski describes amethod for keeping same average dissipating power, but using it moreduring the NMR experiment and less in-between.

[0043]FIG. 7 shows the apparatus of Macovski for conventional MRI or forwide-line NMR analysis of a human body 510. The main field coils areusually a series of solenoids providing a relatively uniform field. Forillustration we show two series connected solenoids 511 and 512. Inconventional systems, these are powered by a d.c. source whose currentis limited by the dissipation of the coils themselves. The coils aredriven by pulse source 513. During the data acquisition period,corresponding to time interval T, the current is increased to I_(peak)as shown. As long as the time T is small compared to the thermal timeconstant or thermal inertial of the coil system, the magnet will notreach its final temperature. Instead the temperature will be based onthe average power input, rather than the peak power input. Therefore, aslong as the duty cycle of the pulses used are relatively low, the peakcurrent I_(peak), and the resultant magnetic field, can be much higherthan is conventionally used, so that the desired range of 0.6-6.0 mT canbe reached with a relatively inexpensive system. RF coils 517 and 518produce a field normal to the main field for exciting the nuclear spinsand receiving the resultant signals. Data acquisition system 514generically represents the transmitter for the RF coils and the receiverof the resultant signals at the appropriate time intervals. Thesesignals are processed by data processing system 515 to obtain an imageof object 510. The results of the analysis are displayed in 516.

[0044] Limitations on use of the method of Macovski for imaging are thehigh requirements for stability that have to be satisfied in order to doimaging and even higher requirements for spectroscopy. To achieve thehighest possible field one should use an iron yoke, which will bringsubstantial non-linearity and hysteresis. An additional problem is therequirement to polarized the subject to a point of saturation. This timeis at least several seconds.

[0045] There are certain medical applications that do not require thehigh magnetic fields and resolution needed in imaging of the human body.Information on body composition is of great interest in many medicaldiagnostic and monitoring applications. Body fat mass and fat free massis of particular importance. DXA is capable of separating fat, fat freemass, and bone mineral and it readily available. However, accuracy andprecision of this technique is not good enough and there is no cleardefinition what is fat tissue. CT, ultrasound and MRI were applied tothe field of tissue composition in distinct anatomical compartments. Inthe field of in-vivo analysis of whole human body composition there havebeen numerous attempts to use Magnetic Resonance methods and techniques.For example, in Lung Water Quantitation by Nuclear Magnetic ResonanceImaging, C. E. Hayes et al, Science, Vol. 216, pp. 1313-5, 1982 usedconventional MRI to quantitatively estimate the water distribution ofsaline-filled and normal rat lungs in isolated and in situ preparations.In Use of NMR for Measurement of Total Body Water and Estimation of BodyFat, D. S. Lewis, J Appl. Physiol., Vol. 60, pp. 836-40, 1986 usedproton MRI to determine total body water in baboons. The hydrogenassociated with water was measured as the amplitude of thefree-induction decay voltage. Body water calculated by multiplying peakamplitude by the experimentally determined constant for a water standardwas similar to that determined gravimetrically in the same baboons.

[0046] Additionally, whole body composition for humans and specially foranimals requires the measurement to be very short as compared to theirmovement. This time may be of an order of several milliseconds andshorter. Similar situation is in the logging environment. Due to highlogging speed pre-polarization is problematic and require very longmagnet by prior art techniques. Without full polarization there is noways to calibrate a logging tool. Working with partially polarizednuclei as proposed in the present invention solves this problem.

[0047] The basic coil and magnet arrangement of Macovski may be used inone embodiment of the present invention for determination of whole bodycomposition. In NMR relaxation measurements, the sample volume is a fewcubic centimeters while is it as small as few cubic millimeters in atypical MRI experiment. For simplicity we may assume thatSignal-to-Noise (S/N) ratio is proportional to sample volume and 7/4power of Larmor frequency. Whole body volume, on the other hand, is ofan order of 110 cubic centimeters. Neglecting Quality Factor (Q),variability, filling factors, sample loading of Q and other secondaryfactors, to keep the same S/N we may reduce frequency by a factor of10³, or approximately from few MHz to few KHz. Hydrogen nuclear magneticresonance in Earth's magnetic field is a few KHz depending on aparticular geographical location. In Self-Diffusion Imaging by Spin Echoin Earth's Magnetic Field, A. Mohorie et al, Journal of MagneticResonance, Vol. 136, pp. 22-26, 1999 used the NMR in the Earth'smagnetic field to test the methods of diffusion-weighted measurements inextremely weak magnetic fields. The main modification that has to bedone to the method of Macovski is a scaling down of the frequencies andthe currents.

[0048] While the foregoing disclosure is directed to the preferredembodiments of the invention, various modifications will be apparent tothose skilled in the art. It is intended that all variations within thescope and spirit of the appended claims be embraced by the foregoingdisclosure.

What is claimed is:
 1. A method for nuclear magnetic resonance (NMR)sensing of earth formations comprising: (a) using an electromagnet on alogging tool in a borehole within the earth formations for inducing atime varying magnetic field for polarization of nuclei within a regionof the earth formations, said electromagnet being used between anactivation time and a deactivation time; (b) generating during a firsttime interval a first radio frequency (RF) magnetic field having a firstfrequency for causing precession of said nuclei, said first frequencybeing related to a first intensity of the static field induced duringsaid first time interval; (c) generating during a second time intervallater than the first time interval a second RF magnetic field having asecond frequency for refocusing said precessing nuclei, said secondfrequency being related to a second intensity of the static fieldinduced during the second time interval; and (d) receiving at a thirdtime interval after the second time interval an NMR signal from saidnuclei at a third frequency, wherein said third frequency is related toa third intensity of the static field induced during said second timeinterval.
 2. The method of claim 1 wherein said first time interval isdelayed relative to said activation time by a wait time related to alongitudinal relaxation time T₁ of material within said region.
 3. Themethod of claim 1 wherein said wait time is at least equal to T₁.
 4. Themethod of claim 1 wherein said first RF magnetic field has a tippingangle substantially equal to 90° and the second RF magnetic field has atipping angle between 90° and 180°.
 5. The method of claim 2 furthercomprising repeating (a)-(c) at least once and acquiring NMR signalswith a plurality of wait times.
 6. The method of claim 1 wherein atleast one of the first, second and third intensities is between 0.6 and6.0 mT.
 7. The method of claim 1 wherein said electromagnet comprises atleast one coil with windings in a plane substantially parallel to alongitudinal axis of said borehole
 8. The method of claim 7 whereingenerating said RF field further comprises using a coil on the loggingtool with windings in a plane substantially parallel to saidlongitudinal axis and substantially parallel to a total dipole moment ofsaid electromagnet.
 9. The method of claim 1 wherein said electromagnetcomprises at least one coil wound with windings in a plane substantiallyperpendicular to a longitudinal axis of said borehole
 10. The method ofclaim 9 wherein generating said first and second RF fields furthercomprises using a coil on the logging tool with windings in a planesubstantially parallel to said longitudinal axis.
 11. The method ofclaim 1 wherein said logging tool is conveyed into the borehole in oneof (i) a wireline, (ii) a drillstring, and, (iii) coiled tubing.
 12. Themethod of claim 1 wherein at least one of the first, second and thirdfrequencies is different from one of the other frequencies.
 13. In amethod of nuclear magnetic resonance (NMR) sensing of earth formations,using, in a borehole containing a ferromagnetic casing and ferromagneticdebris, a logging tool designed to have (i) a permanent magnet forgenerating a static magnetic field in the earth formations, and (ii) atleast one antenna for generating a modulated radio frequency (RF)magnetic field in the earth formations and receiving NMR signalstherefrom, an improvement comprising: (a) replacing said permanentmagnet by an electromagnet operated between an activation time anddeactivation time for inducing the static magnetic field; and (b)initiating a pulse for modulating said RF field at a start time delayedrelative to said activation time by a wait time related to alongitudinal relaxation time T₁ of material within said region.
 14. Themethod of claim 13 wherein said wait time is at least equal to T₁. 15.The method of claim 13 wherein the at least one pulse further comprisesa plurality of pulses forming one of: (i) a CPMG sequence, and, (ii) amodified CPMG sequence.
 16. The method of claim 13 further comprisingrepeating (b) at least once and acquiring NMR signals with a pluralityof wait times.
 17. The method of claim 13 wherein said static magneticfield has a field strength between 0.6 and 6.0 mT.
 18. The method ofclaim 13 wherein said electromagnet comprises at least one coil withwindings in a plane substantially parallel to a longitudinal axis ofsaid borehole
 19. The method of claim 18 wherein said at least oneantenna further comprises a coil on the logging tool with windings in aplane substantially parallel to said longitudinal axis and substantiallyparallel to a total dipole moment of said electromagnet.
 20. The methodof claim 13 wherein said electromagnet comprises at least one coil woundwith windings in a plane substantially perpendicular to a longitudinalaxis of said borehole.
 21. The method of claim 20 wherein said at leastone antenna further comprises a coil on the logging tool with windingsin a plane substantially parallel to said longitudinal axis.
 22. Amethod for nuclear magnetic resonance (NMR) sensing comprising: (a)using a magnet for inducing a static magnetic field for polarization ofspins of nuclei in materials to be analyzed, said static magnetic fieldhaving a time-varying field intensity; (b) generating a radio frequency(RF) magnetic field having a first frequency for causing precession ofspins of said nuclei, wherein said first frequency is generated during afirst time interval and is related to a first intensity of the staticfield induced during said first time interval; and (c) receiving at asecond time interval delayed relative to the first time interval a NMRsignal from said precessing nuclei at a second frequency, wherein saidsecond frequency is related to a second intensity of the static fieldinduced during said second time interval.
 23. The method of claim 22further comprising generating a third RF magnetic field at a third timeinterval between the first and second time intervals for refocusing ofsaid precessing spins.
 24. The method of claim 22 wherein said magnetcomprises an electromagnet.
 25. The method of claim 22 wherein saidmaterials to be analyzed comprises an earth formation, the methodfurther comprising conveying said electromagnet into a borehole in theearth formation.
 26. The method of claim 22 wherein said materials to beanalyzed comprises a human body, the method further comprisingdetermining from said NMR signal a whole body fat-to-lean ratio.
 27. Themethod of claim 25 further comprising calibrating said NMR signal anddetermining a composition of a fluid in the earth formation.
 28. Themethod of claim 22 wherein using said electromagnet further comprisesdischarging a capacitor through the electromagnet.
 29. A method fornuclear magnetic resonance (NMR) sensing of a bulk property of a bodycomprising: (a) using an electromagnet surrounding the body, between anactivation time and deactivation time for inducing a static magneticfield for polarization of nuclei within said body, said static fieldhaving a field strength between 0.6 and 6.0 mT.; (b) generating a radiofrequency magnetic (RF) field for exciting said nuclei, said RF fieldhaving a carrier frequency and being modulated by a pulse sequence; and(c) receiving NMR signals from said nuclei; wherein said pulse sequencecomprises at least one pulse having a start time delayed relative tosaid activation time by a wait time related to a longitudinal relaxationtime T₁ of material within said region.
 30. The method of claim 29wherein said wait time is at least equal to T₁.
 31. The method of claim29 wherein the at least one pulse further comprises a plurality ofpulses forming one of: (i) a CPMG sequence, and, (ii) a modified CPMGsequence.
 32. The method of claim 29 further comprising repeating(a)-(c) at least once and acquiring NMR signals with a plurality of waittimes.
 33. The method of claim 29 wherein said bulk property furthercomprises fat mass and fat-free mass of the body.
 34. A well loggingapparatus for nuclear magnetic resonance (NMR) sensing of earthformations comprising: (a) an electromagnet on a logging tool in aborehole within the earth formations for inducing a time varying staticmagnetic field for polarization of nuclei within a region of the earthformations, said electromagnet being used between an activation time anda deactivation time; (b) a transmitter for inducing: (A) during a firsttime interval a first radio frequency (RF) magnetic field having a firstfrequency for causing precession of said nuclei, said first frequencybeing related to a first intensity of the static field induced duringsaid first time interval; and (B) during a second time interval laterthan the first time interval a second RF magnetic field having a secondfrequency for refocusing said precessing nuclei, said second frequencybeing related to a second intensity of the static field induced duringthe second time interval; and (c) a receiver for receiving at a thirdtime interval after the second time interval an NMR signal from saidnuclei at a third frequency, wherein said third frequency is related toa third intensity of the static field induced during said second timeinterval.
 35. The apparatus of claim 34 wherein said first RF magneticfield has a tipping angle substantially equal to 90° and the second RFmagnetic field has a tipping angle between 90° and 180°.
 36. Theapparatus of claim 34 wherein said pulse sequence comprises one of (i) aCPMG pulse sequence, and, (ii) a modified CPMG pulse sequence.
 37. Theapparatus of claim 34 wherein at least one of the first, second andthird frequencies is different from at least one of the other twofrequencies.
 38. The apparatus of claim 34 wherein at least one of thefirst, second and third intensities is between 0.6 and 6.0 mT.
 39. Theapparatus of claim 34 wherein said electromagnet further comprises atleast one coil with windings in a plane substantially parallel to alongitudinal axis of said borehole.
 40. The apparatus of claim 39wherein said transmitter further comprises at least one coil withwindings in a plane substantially parallel to a longitudinal axis ofsaid borehole.
 41. The apparatus of claim 34 wherein said electromagnetfurther comprises at least one coil wound with windings in a planesubstantially perpendicular to a longitudinal axis of said borehole. 42.The apparatus of claim 41 wherein said transmitter further comprises acoil on the logging tool with windings in a plane substantially parallelto said longitudinal axis.
 43. The apparatus of claim 34 furthercomprising a capacitor operatively coupled to the electromagnet, saidelectromagnet producing said time varying statice field upon dischargeof the capacitor therethrough.